I need help with math

How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

The function a represents the cost of manufacturing product a, in hundreds of dollars, and the function b represents the cost of manufacturing product b, in hundreds of dollars. a(t) = 5t + 2 b(t) = 7t - 2t +4 find the expression that describes the total cost of manufacturing both products, a(t) + b(t). a. 7t^2 - 7t + 2 b. 7t^2 + 3 +6 c. 7t^2 + 7t - 6 d. 7t^2 - 3t + 6

65% of students in your school participate in at least one after school activity. if there are 980 students in you school, how many do not participate in an after school activity?